The present invention relates, in general, to electronics and, more particularly, to methods of forming semiconductor devices and structure.
In the past, various circuits and methods were used to control synchronous rectifiers in power supply systems. Usually, the control methods depended on the operating mode and the type of power supply system. Flyback type power supply systems usually required complex control circuits. In a flyback converter, the current through a primary coil of a transformer was terminated in order to cause the magnetic field to collapse and couple power to a secondary inductor of the transformer. In the flyback converter, the synchronous rectifier typically was in the secondary side of the power supply system and the switching power supply controller was in the primary side of the power supply system. One method of operating a flyback converter system utilized a fixed frequency clock to predict the time in which the synchronous rectifier should be enabled or disabled. An example of such a flyback system was disclosed in U.S. Pat. No. 6,418,039 issued to Franco Lentini et al. on Jul. 9, 2002. Complicated circuitry was required to implement the control, which increased the system cost. Additionally, some flyback systems also included burst-mode operation to reduce power dissipation during light load conditions. It was difficult to accurately predict the proper time to enable and disable the synchronous rectifier, thus, the operation was inefficient.
Accordingly, it would be advantageous to have a control method and circuit that accurately controls a secondary side synchronous rectifier that does not include complicated circuitry. It is desirable for the circuit and method to be cost and time efficient to implement.
For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference characters in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or an anode of a diode, and a control electrode means an element of the device that controls current flow through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain n-channel or p-channel devices, or certain n-type or p-type doped regions, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with embodiments of the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action and the initial action. The use of the words approximately, about, or substantially means that a value of an element has a parameter that is expected to be very close to a stated value or position. However, as is well known in the art there are always minor variances that prevent the values or positions from being exactly as stated. It is well established in the art that variances of up to about ten percent (10%) (and up to twenty percent (20%) for semiconductor doping concentrations) are regarded as reasonable variances from the ideal goal of exactly as described.
It should be noted that a logic zero voltage level (VL) is also referred to as a logic low voltage or logic low voltage level and that the voltage level of a logic zero voltage is a function of the power supply voltage and the type of logic family. For example, in a Complementary Metal Oxide Semiconductor (CMOS) logic family a logic zero voltage may be thirty percent of the power supply voltage level. In a five volt Transistor-Transistor Logic (TTL) system a logic zero voltage level may be about 0.8 volts, whereas for a five volt CMOS system, the logic zero voltage level may be about 1.5 volts. A logic one voltage level (VH) is also referred to as a logic high voltage level, a logic high voltage, or a logic one voltage and, like the logic zero voltage level, the logic high voltage level also may be a function of the power supply and the type of logic family. For example, in a CMOS system a logic one voltage may be about seventy percent of the power supply voltage level. In a five volt TTL system a logic one voltage may be about 2.4 volts, whereas for a five volt CMOS system, the logic one voltage may be about 3.5 volts.